Using capillary gas chromatography, we analyzed the fatty acid composition of human milk from 23 women who had delivered prematurely and 16 women who had delivered at term. Milk samples were obtained at 1, 2, 4, 12, and 26 weeks after delivery. The relative amounts of saturated and monounsaturated fatty acids in preterm and term milk remained stable throughout the 6 months of lactation. The proportions of linoleate (18:2n-6) and alpha-linolenate (18:3n-3) were similar in preterm and term milk and showed an increasing trend from transitional (8.7-9.9% and 0.9-1.1% of total fatty acids, respectively) to mature milk (9.9-11.8% and 1.2-1.5%, respectively). The proportions of the major long-chain polyunsaturated fatty acids (LCP), 20:3n-6, 20:4n-6, 22:5n-3 and 22:6n-3, were highest at 1 week and decreased thereafter in both types of milk. In term milk, the proportion of LCP continued to decrease from 1 month to 6 months, whereas in preterm milk it was fairly constant. Consequently, at 6 months of lactation, the relative content of arachidonate (20:4n-6) was 1.5 times (p < 0.05) and that of docosahexaenoate (22:6n-3) was two times higher (p < 0.01) in preterm than in term milk. We conclude that in long-term lactation, preterm human milk provides a significantly higher relative supply of LCP than term human milk. This higher LCP content may be of special benefit to the development of a preterm infant.